A surface acoustic wave travelling on the optically polished surface of an elastic substrate is superior in various ways to a bulk wave that was conventionally used, and is more and more used in a variety of electronic parts such as filters. FIG. 1 shows an example of a filter in which reference numeral 1 denotes a piezoelectric substrate, 2 refers to an input transducer comprising a pair of interdigital electrodes 3A and 3B, and 4 to an output transducer comprising a pair of interdigital electrodes 5A and 5B, respectively. A signal supplied through an input terminal IN is converted to a surface acoustic wave by the input transducer 2 and travels on the surface of the elastic substrate 1 in the arrow direction in the Figure. When the surface acoustic wave reaches the output transducer 4, it is converted to an electric signal and is taken out from an output terminal OUT.
The filter of FIG. 1, however, wherein two transducers 2 and 4 each including the pair of interdigital electrodes 3A and 3B or 5A and 5B are provided, cannot be free from electromechanical conversion loss because the transducers 2 and 4 operate to propagate surface acoustic waves in both right and left directions, respectively.
To alleviate the drawback, there was proposed a unidirectional transducer which propagates a surface acoustic wave in only one direction on the surface of a piezoelectric substrate. It is known to use in such a unidirectional transducer a 120.degree. phase shifter or 90.degree. phase shifter, as shown in FIG. 2, or otherwise a reflector as shown in FIG. 3.
In FIG. 2, reference numerals 6, 6A and 6B are electrodes with 120.degree. phase difference relative to each other. The electrodes 6 and 6A are kept uncontacted by gaps 7 or insulative layers provided therebetween. Thereby, a surface acoustic wave travels in only one direction.
However, phase shifters which require specific provision at intersections of electrodes to prevent contact therebetween make manufacturing processes complicated.
In FIG. 3, reference numerals 8A and 8B are a power supply section and a reflecting section, both being of a normalized type and including interdigital electrodes.
Reference numeral 9 refers to a common electrode having portions 8A and 8B, 10 to a signal source, 11 to a matching circuit, and 12 to a reactance circuit, respectively. With this arrangement, a signal applied from the signal source 10 through the matching circuit 11 is converted to surface acoustic wave and travels in both the right and left directions from the power supply section 8A. The wave which traveled to the left is reflected and returned to the right by the reflecting section 8B to which the reactance circuit 12 is connected. The reflected wave is thereby synthesized in the power supply section 8A with the wave travelling to the right. In this case, when both waves are near the center frequency of the device, they substantially coincide in their phases. However, when both waves are far from the center frequency, they compensate or in other words partially cancel each other. As the result, the surface acoustic wave also travels in the contrary direction and the aimed unidirectional propagation cannot be attained. This means that the surface-acoustic-wave device is not suitable for wide band wave processing.